Existing and emerging indoor/microcellular systems (such as wireless PBX, private wireless networks on campus, in buildings or factories) often try to autonomously reuse the channels allocated to the outdoor/macrocellular systems. The prevention of mutual interference between an indoor microcell and the outdoor macrocells is easily achievable with the conventional Fixed Channel Assignment (FCA) where only a part of the whole spectrum is assigned to each of the outdoor macrocells.
An indoor microcell popping up within an outdoor macrocell cell or at the boundaries common to two or more macrocells has plenty of spectrum for utilization without mutual interference with the outdoor mobile conversations. With proper settings of power levels and the choice of frequencies for indoor cells the two systems can be set for interference free operation.
Dynamic Channel Assignment (DCA) is being adopted to enhance the spectral utilization and to ease the frequency planning in wireless networks. For example, see the articles by Chih-Lin I. and P. Chao, "Local Packing-Distributed Dynamic Channel Allocation at Cellular Base Station," Proc. GLOBCOM 1993 and "Local Packing-Distributed Dynamic Channel Allocation with Cosite Adjacent Channel Constraints," Proc. IEEE PIMRC 1994 and the article by M. Haleem, K. Cheung, and J. Chuang, "Aggressive Fuzzy Distributed Dynamic Channel Assignment for PCS," Proc. ICUPC '95, pp.76. These DCA algorithms range from simple selection of a feasible channel (S-DCA), to maximal packing where a call request is rejected only when there is no feasible channel with all possible rearrangements.
The advantage of DCA lies in that every cell is free to choose any channel from the universal set of channels available to the network, the only constraint is being imposed through the interference from the cells within the frequency reuse distance. This capability provides capacity gain in addition to alleviating radio frequency planning. In contrast to the above advantages, microcells at a given location in the macrocellular radio environment with DCA may experience interference over any part of the allocated frequency spectrum. This phenomenon may inhibit the indoor autonomous wireless system from finding available channels in real time. The coordination required to avoid mutual interference becomes challenging.